Passive low frequency RFID readers and tags use operating principles that are well-known to those of ordinary skill in the art, and that are described in extensive detail in several seminal inventions, including U.S. Pat. No. 1,744,036 to Brard (1930), U.S. Pat. No. 3,299,424 to Vinding (1967), U.S. Pat. No. 3,713,146 to Cardullo (1973), and U.S. Pat. No. 5,053,774 to Schuermann (1991), and in textbooks such as “RFID Handbook” (Finkenzeller—1999).
In an inductively coupled RFID system, as depicted in FIG. 1A, the reader includes electronic circuitry, which generates an activation signal (usually a single frequency unmodulated signal) using a signal source [101] and an amplifier [102] to drive a resonant antenna circuit [103]. This activation signal is manifested as a time-varying electromagnetic field [104b], which couples with the ID tag [105] by means of the electromagnetic field's magnetic field component [104a], [104c]. The ID tag [105] converts this magnetic field into an electrical voltage and current, and uses this electrical power to activate its internal electronic circuitry. Using any of several possible modulation schemes, such as amplitude modulation [104d], the ID tag conveys binary encoded information stored within it back to the reader via the magnetic field [104a], [104c] where the detector and utilization circuit [106] converts this binary code into an alphanumeric format tag data [107] in accordance with some prescribed application.
Typically in RFID systems, the resonant antenna circuit [103] includes the circuit elements shown in FIG. 1B and includes a capacitor [108] and an inductor [109], the inductor existing in a physical form such that it produces a spatially distributed magnetic field, and the values of the capacitor and inductor selected so as to resonate at the signal source frequency [101]. Such an inductor commonly is shaped in the form of a closed loop [112] as shown in FIG. 1C, having one or more concentric turns of a conductor [110] and two connecting terminals [111] through which the driving signal is applied.
ISO Standard 11785
International Standards Organization (ISO) Standard 11785, “Radio frequency identification of animals—Technical Concept” (1996) (ISO 11785) defines the technical principles for communications between RFID readers and two types of electronic passive identification transponders. The two types of ID tag technologies defined in ISO 11785 are are designated “full-duplex” (“FDX-B”) and “half-duplex” (“HDX”). Both types of ISO tags, which can also be referred to as ISO transponders, contain identification information stored in binary form that can be conveyed to a companion reader when the transponder is suitably activated by the reader. Additional technical details are provided in ISO Standard 11785, which is incorporated into this disclosure by reference.
Radio frequency identification (RFID) systems that communicate with ISO 11785 transponders typically are low frequency reader devices that generate a tag activation signal, and in response, receive an identification data signal transmitted from the RFID tag. Such a reader can use separate transmit and receive antenna elements to perform these functions. However, readers in which a single antenna performs both transmit and receive functions are very economical and efficient, and are frequently utilized in low-frequency RFID readers.
For both types of passive tags specified in ISO 11785, activation energy is transferred to the tag from the reader, and identification code information is transferred to the reader from the tag through mutual coupling via a magnetic field. The FDX-B tag amplitude modulates the reader's activation signal with its binary identification code sequence. Amplitude modulation imposes variations on the activation signal's magnitude, as illustrated by waveform [104d], and the reader is equipped with sensing circuitry capable of detecting these magnitude variations. These magnitude variations have a specific pattern associated with the tag's embedded binary sequence of ones and zeroes. The reader detects this modulation and derives from it the FDX-B tag's identification code. The term “full-duplex” is indicative that the FDX-B tag sends its identification code information during the time when it is receiving the activation signal from the reader.
In contrast, the HDX tag uses the reader's activation signal to charge an internal capacitor (which functions as a very small rechargeable battery), and the tag uses this stored energy to power a transmitter. Once the activation signal ceases, the HDX transponder emits a very small strength internally generated radio signal, utilizing a frequency shift keyed (“FSK”) modulation scheme. Specifically, the binary identification code information contained in the HDX tag is serially output such that the occurrence of a binary “1” results in the HDX tag's radio signal being 124.2 KHz and a binary “0” results in the tag's radio signal being 134.2 KHz. The reader detects this FSK signal and derives from it the HDX tag's identification code. The term “half-duplex” is indicative that the reader and the HDX tag exchange the activation signal and the identification code signal in alternating time intervals.
An ISO compliant reader has the capability to activate and detect both FDX-B and HDX type tags. To accomplish this, the ISO compliant reader transmits an activation signal, consisting of a 134.2 kilohertz (KHz) sinusoid, which is switched ON and OFF in a prescribed pattern in accordance with ISO 11785. During the interval in which the 134.2 KHz signal is ON, the FDX-B tag is activated and it transmits its identification code signal cyclically for as long as the activation signal is present. During this ON interval also, an HDX tag charges its internal capacitor. Subsequently, during the interval in which the 134.2 KHz activation signal is OFF, the FDX-B tag is dormant, and the HDX tag transmits its identification code sequence a single time.
FIGS. 2A and 2B illustrate the frequency spectral characteristics of the RFID system pertaining to ISO 11785 and related to the present invention. FIG. 2A shows the spectra for the HDX case, where the activation signal [20] appears at 134.2 KHz, and where the HDX tag frequencies appear at 124.2 KHz [21] and 134.2 KHz [22]. Since the activation signal and the HDX transponder signals alternate in time, the 134.2 KHz activation signal [20] and the 134.2 KHz transponder signal [22] do not occur simultaneously. Thus, the reader's receive circuitry is able to detect the transponder frequency without being overwhelmed with its own activation signal.
FIG. 2B shows the spectra for the FDX-B case, where the activation signal [23] appears at 134.2 KHz, and where the FDX-B transponder's amplitude modulation appears as sidebands close to 134.2 KHz [24],[25]. As is well known to those of ordinary skill in the art, amplitude modulation sidebands appear symmetrically around the modulated carrier signal, and for FDX-B specifically, these sidebands appear at ±2.097 KHz and ±4.194 KHz. Because the activation signal and the data signal are distinct frequencies, they can occur simultaneously, and the reader is able to separate the two signals, and recover the data contained in these sideband frequencies. When amplitude modulation signals are demodulated, the upper sideband frequencies are translated downward in frequency and appear at 2.097 KHz and 4.194 KHz [26].
RFID systems, such as systems compliant with ISO 11785, are based on radio frequencies (RF), and are susceptible to electromagnetic interference in the same way that radio communications, such as AM and FM broadcast radio and TV, are affected. Radio frequency interference can disrupt the efficacy of RFID systems by reducing the operating range of tags, and in extreme cases rendering the system inoperative. Because HDX and FDX-B tags use different communicating frequencies and modulation techniques, they can be affected differently by a particular interfering signal. Some types of electromagnetic interference is sufficiently broadband as to affect both HDX and FDX-B indiscriminately, while other types of interference will affect one more than the other. For example, HDX tag signals can be affected by activation signals transmitted from other readers operating nearby, by computer screen backlighting circuitry, and/or by switched mode power device radiation in the range of 100 KHz to 150 KHz. Such sources of interference typically will not interfere with FDX-B tags. FDX-B tags, however, can be affected by electrostatic discharge, reader power supply noise, and RF signals containing FDX-B data frequency amplitude modulation. Both HDX and FDX-B are equally disrupted by AC motor variable frequency drive controllers, which tend to radiate wideband amplitude and frequency modulated RF.
Multi-Loop Antennas
Multi-loop antennas with signal cancelling characteristics are known in the art, and are disclosed in U.S. Pat. No. 4,243,980 to Lichtblau (Jan. 6, 1981). Lichtblau discloses an antenna arrangement applicable to an electronic article surveillance (EAS, or anti-pilferage) system in which two multi-loop antennas are used as separate transmit antenna and receive antenna. Lichtblau discloses multi-loop antennas constructed with two loops and with three loops lying in a common plane, in which each loop is twisted 180° with respect to each adjacent loop so as to be in phase opposition. The presence of an anti-pilferage tag induces a change in the coupling between the transmit antenna and the receive antenna such that the tag is always detectable. Lichtblau's multi-loop transmit antenna produces high intensity near field signals that cancel at distances an order of magnitude larger than the antenna height. In a reciprocal manner, Lichtblau's identical multi-loop receive antenna is receptive to signals from the transmit antenna coupled directly and indirectly coupled via the tag, but it cancels interfering signals that originate at distances large compared to the antenna height.
In a later related patent, U.S. Pat. No. 4,751,516 to Lichtblau (Jun. 14, 1988) therein is disclosed a three loop antenna in which the center loop is twisted 180° with respect to each adjacent loop so as to be in phase opposition, and where the specifications for each loop includes its respective dimensions (e.g., area), number of conductor turns, and current. Thus, the same signal cancelling effect obtained with an antenna comprising a center loop whose area equals the sum of the areas of the two adjacent outer loops, having uniform current and number of conductor turns, can also be obtained with an antenna have non-uniform current, unequal number of conductor turns, and unequal areas, as long as the arithmetic product of these three factors (e.g., current×turns×area) remains constant. For example, the area sum of the two outer loops can be hall that of the center loop, if there are twice as many turns or twice the current in the outer loops than the center loop has.
Lichtblau's multi-loop antenna system, however, uses separate transmit and receive antennas in which the transmit antenna emits a swept frequency signal that causes a passive resonant circuit to produce a magnetic field disturbance that the receive antenna is capable of sensing. Lichtblau's objectives are to produce a very high intensity tag activation field close by the antenna that cancels at distances so as not to exceed allowable regulatory limits on transmitting power and so as not to interfere with other nearby EAS systems, and a receive antenna that is insensitive to other EAS systems and distant active electrical noise sources.
Acoustic Noise Interference
A curious and somewhat enigmatic source of interference that affects principally FDX-B type tags is acoustic noise produced by metallic structures located near reader antennas. In some situations, nearby metallic structures resonate due to machinery vibration and shock. Animal movement can also induce acoustic noise resulting from the impact and vibration of metallic structures that comprise gates, corrals, stanchions, traders, and the like. The deteriorative effect that acoustic noise has on RIFD system efficacy has been documented in several field trials and studies.
In 2007, Scotland's Environment and Rural Affairs Department identified acoustic interference as a substantial obstacle to the successful deployment of a radio frequency identification based animal traceability system, and released a tender (Investigating the Effect of Acoustic/Mechanical Interference on Radio Frequency Identity (RFID) Systems Used to Identify Animals Electronically—Reference CR/2007/01) that offered research project funding to qualified contractors in return for a technical solution and intellectual property rights.
A research study was conducted at Kansas State University in 2007 on a livestock trailer mounted reading system (Trailer Mounted RFID Reader Scans EID Tags During Cattle Shipments). However, during the study many tags were readable less than half the time, and overall tags were readable less than two-thirds of the time. Such low read rates are generally unacceptable in commercial applications, where read rates in excess of 99% are desirable.
Electromagnetic radiation is typically produced by any electrical or electronic device that uses alternating electrical currents in its operation. Emitted electromagnetic radiation can create interference among electronic devices that share the same spectrum. In some instances, electromagnetic interference can be effectively filtered electronically at the source that creates it or at the device being disrupted by it, using passive or active electronic circuits, by using computational techniques such as digital signal processing, or by using metallic shields that block or redirect the radiation. Electromagnetic interference is especially problematic when the interfering frequencies coincide with the frequencies used by a particular radio frequency system, and separating the desired signal from the interfering signal can be particularly difficult.